The microtubule associated protein tau is essential for development and maintenance of the nervous system. On the other hand, tau dysfunction has long been correlated with Alzheimer?s disease pathology. Further, recent genetic evidence demonstrates that mutations affecting either the primary structure of tau or the regulation of tau RNA alternative splicing lead to pathological tau fiber formation similar to that observed in Alzheimer?s disease, neuronal cell death and FTDP-17, a collection of human dementias. Gaining a thorough understanding of tau action is therefore an important goal. Unfortunately, our understanding of tau action is derived primarily from in vitro biochemical studies. It is essential that conclusions drawn from these in vitro studies be assessed and extended in cellular systems. The problem is that all currently available systems to study tau action in cells suffer from major inherent pragmatic or technical limitations. Here, we propose to adapt a cell system with a long and productive history in cell and developmental biology, the echinoderm egg, to overcome many of these limitations. The principle of the assay is that tau, microinjected into eggs prior to fertilization, will inhibit normal fertilization induced microtubule dependent events (such as cell division) in a dose dependent manner, but not microtubule independent events. This assay is rapid, flexible and quantitative - this combination of critical features is not available in any other cellular system to study tau action upon microtubules. For simplicity, we refer to this assay as the Inhibition of Early Development Bioassay, or "IEDB."Using this system, we will (i) test the predictions of in vitro mechanistic studies of tau structure-function and (ii) investigate molecular mechanisms of tau action.